1. Field of the Invention
This invention pertains to navigation systems and more particularly to a vehicle navigation system for selecting a route on roads and highways and even more particularly to routing to a destination at an intersection of two roads using a road segment based database.
2. Description of the Prior Art
Vehicle navigation systems are well known; see Yamada, U.S. Pat. No. 4,926,336 incorporated herein by reference disclosing a route searching system using a database describing road information including road intersections. See also Link et al. U.S. Pat. No. 5,270,937, incorporated herein by reference, disclosing a vehicle navigation/position system which searches road segments stored in a road map data memory for determining vehicle position in respect to roadway map coordinates.
Vehicle navigation systems require that road information be stored in a database. The road information is often in the form of road segments, where a road segment is a length of a road between two nodes. Typically a node is an intersection with another road or with an artificial boundary such as a state boundary. The information stored in the database describes each road in a particular geographical area for purposes of. route guidance and display of a map showing the roads.
Two main formats are used for storage of road information in the database. The first format, typically favored in the United States for instance, is road segment based database. In this case the information pertains to the road segments.
The other format is an intersection (node) based database where the emphasis of the stored information is on the nodes (intersections) themselves and the roads are defined as the connections between nodes. It is not unusual that for a particular geographic area only one type of database is available. This is often because such databases are provided by third party commercial services and it is sometimes not economically feasible to provide both types of databases for a particular geographical area. An example of a node based database for a navigation system is disclosed in Ichikawa U.S. Pat. No. 5,410,485 assigned to Alpine Electronics Inc., incorporated herein by reference; see FIG. 20. An example of a segment based database for a navigation system is disclosed in Loughmiller, Jr. et al. U.S. Pat. No. 4,914,605 assigned to ETAK Inc., incorporated herein by reference, see column 12 line 46 and following and FIG. 6A.
Hence it is not unusual for a navigation system to be required to operate with a database which is segment oriented. Such a navigation system in most cases operates satisfactorily; however the present inventor has determined that in certain circumstances suboptimum performance is provided when the destination is at a road intersection.